Electroluminescent display panels have gained acceptance as alphanumeric displays for portable computers and for other portable systems requiring displays because of their small size, light weight and desirable display characteristics. Electroluminescent (EL) display panels are not as large as cathode ray tubes and do not have the disadvantages of liquid crystal displays such as a small viewing angle and sensitivity to ambient lighting conditions. Electroluminescent display panels are also used in automobiles and aircraft cockpits.
Electroluminescent display panels are typically configured as a two-dimensional array of individual EL devices, or pixels, each having a row connection and a column connection. The individual devices include a light emitting phosphor layer, typically of manganese doped zinc sulfide, sandwiched between dielectric layers. Electrodes for applying energizing voltages, including at least one transparent electrode, are attached to the dielectric layers. In order to produce light, the individual EL devices require the application of a voltage above a threshold magnitude and having transitions of alternating polarity. The resultant light output requires approximately 0.5 to 1 millisecond to decay to 1/e of peak brightness following an applied voltage pulse. EL display panels are typically time multiplexed by rows and are operated at a 60 Hz refresh rate.
The first generation of thin film electroluminescent drive electronics utilized scanning of one row of the display at a time at a sufficiently high frame rate to reproduce a visible display on the panel. As each row is addressed, selected pixels are addressed in parallel on column electrodes. A prior art asymmetric drive technique provided the EL panel with alternating polarity drive pulses by applying a negative subthreshold voltage to one row at a time. During each row scan time, a positive voltage pulse is applied to the selected columns, and zero voltage is applied to the nonselected columns. At the intersection of the selected columns and the selected row, the pixel receives the voltage necessary for light emission. At the intersection of the nonselected columns, the pixels are at or below the threshold voltage and do not emit light. After all rows of the panel have been addressed, a positive polarity refresh pulse is applied to all of the rows simultaneously, and all columns are held at zero volts potential. The asymmetric drive technique provides two peaks of light from each selected pixel for each frame of the display. For the typical 60 Hz refresh rate, light is emitted from selected pixels at a 120 Hz rate.
The disadvantage of the asymmetric drive technique is that a d.c. net charge results on nonselected pixels. The net charge over a period of time produces permanent damage to the display. A fixed pattern displayed for a long period can produce a change in the pixel threshold voltage versus brightness, known as differential aging. When this occurs, previously selected pixels become brighter than nonselected pixels, resulting in a retained image. The retained image is undesirable to users.
To reduce the effect of retained image, a symmetrical drive scheme was developed. In the symmetrical drive scheme, the refresh pulse is eliminated and alternating polarity drive pulses are applied to the panel. To maintain alternating polarity drive, the rows are scanned with pulses of alternating polarity on even and odd frames. The alternating polarity produces a net zero charge on all display pixels, thereby reducing retained image. However, since the refresh pulse is eliminated, light pulses from selected pixels occur at a 60 Hz rate, and the brightness of the display is reduced by 50%.
One way to increase the light output of an EL display panel is to increase the refresh rate of the display so that the average light output perceived by the viewer is increased. This approach has several disadvantages. The standard refresh rate utilized by computers is 60 Hz. To increase the refresh rate would require storage in a semiconductor memory circuit of the 60 Hz frame data received from the computer. In addition, extensive circuitry is required for retrieval and display of the stored data. The extra circuitry complicates the panel assembly and adds to its cost.
It is a general object of the present invention to provide improved methods and apparatus for energizing electroluminescent display devices and panels.
It is another object of the present invention to provide electroluminescent display apparatus with a high light output level.
It is a further object of the present invention to provide electroluminescent display apparatus having little or no retained image.
It is yet another object of the present invention to provide methods and apparatus for energizing electroluminescent display devices utilizing a burst of energizing pulses to increase output brightness.
It is still another object of the present invention to provide methods and apparatus for operating electroluminescent display panels with increased brightness at a refresh rate of 60 Hz.